Avian predators taste-reject aposematic prey on the basis of their chemical defence.

Avian predators learn to avoid defended insects on the basis of their conspicuous warning coloration. In many aposematic species, the level of chemical defence varies, with some individuals being more defended than others. Sequestration and production of defence chemicals is often costly and therefore less defended individuals enjoy the benefits of the warning signal without paying the full costs of chemical production. This is a fundamental theoretical problem for the evolutionary stability of aposematism, since less defended individuals appear to be at a selective advantage. However, if predators sample aposematic prey and selectively reject individuals on the basis of their chemical investment, aposematism could become evolutionarily stable. Previous research aimed at testing whether birds can use taste to discriminate between palatable and unpalatable prey has been confounded by other experimental factors. Here, we show that birds can taste and reject prey entirely on the basis of an individual's level of chemical defence and more importantly, they can make decisions on whether or not to consume a defended individual based upon their level of chemical investment. We discuss these results in relation to the evolution of aposematism, mimicry and defence chemistry.